U.S. patent application number 13/223754 was filed with the patent office on 2012-09-06 for cathode materials having high energy density and lithium secondary battery containing the same.
This patent application is currently assigned to LG CHEM, LTD.. Invention is credited to Seungeun CHOI, Eunyoung GOH, Heegyoung KANG, Kiwoong KIM, Hyang Mok LEE, Sangbaek RYU.
Application Number | 20120225343 13/223754 |
Document ID | / |
Family ID | 42710106 |
Filed Date | 2012-09-06 |
United States Patent
Application |
20120225343 |
Kind Code |
A1 |
CHOI; Seungeun ; et
al. |
September 6, 2012 |
CATHODE MATERIALS HAVING HIGH ENERGY DENSITY AND LITHIUM SECONDARY
BATTERY CONTAINING THE SAME
Abstract
Disclosed is a cathode material comprising a mixture of an oxide
powder (a) defined herein and an oxide powder (b) selected from the
group consisting of an oxide powder (b1) defined herein and an
oxide powder (b2) defined herein and a combination thereof wherein
a mix ratio of the two oxide powders (oxide powder (a): oxide
powder (b)) is 50:50 to 90:10. The cathode material uses a
combination of an oxide powder (a) and 50% or less of an oxide
powder (b) which can exert high capacity, high cycle stability,
superior storage stability and high-temperature stability, thus
advantageously exhibiting high energy density and realizing high
capacity batteries.
Inventors: |
CHOI; Seungeun; (Daejeon,
KR) ; GOH; Eunyoung; (Daejeon, KR) ; LEE;
Hyang Mok; (Daejeon, KR) ; KANG; Heegyoung;
(Cheonan-si, KR) ; RYU; Sangbaek; (Daejeon,
KR) ; KIM; Kiwoong; (Daejeon, KR) |
Assignee: |
LG CHEM, LTD.
Seoul
KR
|
Family ID: |
42710106 |
Appl. No.: |
13/223754 |
Filed: |
September 1, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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PCT/KR2010/001305 |
Mar 3, 2010 |
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13223754 |
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Current U.S.
Class: |
429/136 ;
252/519.15 |
Current CPC
Class: |
H01M 4/623 20130101;
H01M 4/364 20130101; H01M 4/131 20130101; H01M 2/1646 20130101;
Y02T 10/70 20130101; H01M 2/1673 20130101; H01M 2004/028 20130101;
Y02E 60/10 20130101; H01M 2/1653 20130101; H01M 4/525 20130101;
H01M 4/505 20130101; H01M 2/0287 20130101; H01M 2/166 20130101;
H01M 4/622 20130101; H01M 2/1633 20130101; H01M 10/052
20130101 |
Class at
Publication: |
429/136 ;
252/519.15 |
International
Class: |
H01M 2/18 20060101
H01M002/18; H01B 1/08 20060101 H01B001/08 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 3, 2009 |
KR |
10-2009-0018123 |
Claims
1. A cathode material comprising a mixture of an oxide powder (a)
defined below and an oxide powder (b) selected from the group
consisting of an oxide powder (b1) defined below and an oxide
powder (b2) defined below and a combination thereof wherein a mix
ratio of the two oxide powders (oxide powder (a): oxide powder (b))
is 50:50 to 90:10. [Oxide Powder (a)] An oxide powder represented
by Formula 1 below: Li.sub.x(Co.sub.yA.sub.mD.sub.z)O.sub.t (1)
wherein 0.8.ltoreq.x.ltoreq.1.2, D.ltoreq.z.ltoreq.0.3,
1.8.ltoreq.t.ltoreq.4.2, (0.8-m-z).ltoreq.y.ltoreq.(2.2-m-z),
0.ltoreq.m.ltoreq.0.3, A is at least one selected from Mg and Ca,
and D is at least one selected from the group consisting of Ti, Zr,
Hf, V, Nb, Ta; Cr, Mo, W, Mn, Tc, Re, Fe, Ru, Os, Co, Rh, Ir, Ni,
Pd, Pt, Cu, Au, Ag, Zn, Cd, Hg, B, Al, Ga, In, TI, C, Si, Ge, Sn,
Pb, N, P, As, Sb and Bi; [Oxide Powder (b1)] An oxide powder
represented by Formula 2a below:
Li.sub.x(Ni.sub.1-a-bMn.sub.aCo.sub.b).sub.yO.sub.2 (2a) wherein
0.05.ltoreq.a.ltoreq.0.4, 0.1.ltoreq.b.ltoreq.0.4,
0.4.ltoreq.1-a-b.ltoreq.0.7, 0.95.ltoreq.x.ltoreq.1.05,
1.9.ltoreq.x+y'2.3; [Oxide Powder (b2)] An oxide powder which
contains a transition metal mixture of Ni, Mn and Co, has an
average oxidation number of all transition metals except for
lithium, larger than +3 and satisfies Equations 3 and 4 below:
1.1<m(Ni)/m(Mn)<1.5 (3) 0.4<m(Ni.sup.2+)/m(Mn.sup.4+)<1
(4) wherein m(Ni)/m(Mn) is a molar ratio of nickel to manganese and
(Ni.sup.2+)/m(Mn.sup.4+) is a molar ratio of Ni.sup.2+to
Mn.sup.4+.
2. The cathode material according to claim 1, wherein a mix ratio
of the oxide powders is 50:50 to 70:30.
3. The cathode material according to claim 1, wherein the oxide
powder (a) is LiCoO.sub.2.
4. The cathode material according to claim 1, wherein, in the oxide
powder (b1), lithium ions are intercalated and deintercalated
between mixed transition metal oxide layers ("MO layers") and some
Ni ions derived from MO layers are inserted into intercalation and
deintercalation layers of the lithium ions ("reversible lithium
layer") to bond the MO layers.
5. The cathode material according to claim 4, wherein Ni.sup.2+ and
Ni.sup.3+ are present together in the MO layers, and some Ni.sup.2+
is inserted into the reversible lithium layer.
6. The cathode material according to claim 5, wherein a molar
fraction of Ni.sup.2+ inserted into the reversible lithium layer in
the oxide powder (b1) is 0.03 to 0.07, based on the total weight of
transition metals of the oxide powder (b1).
7. The cathode material according to claim 1, wherein, in the oxide
powder (b2), m(Ni)/m(Mn) satisfies
1.2.ltoreq.m(Ni)/m(Mn).ltoreq.1.4.
8. The cathode material according to claim 1, wherein, in the oxide
powder (b2), the average oxidation number of transition metals
other than lithium is higher than 3.0 and is lower than or equal to
3.5.
9. The cathode material according to claim 1, wherein the nickel in
the oxide powder (b2) is composed of nickel (1) present in an
excessive amount, as compared to the manganese content, and nickel
(2) present in an amount corresponding to the manganese
content.
10. The cathode material according to claim 1, wherein the nickel
in the oxide powder (b2) has an average oxidation number higher
than 2+.
11. The cathode material according to claim 9, wherein the nickel
(1) is Ni.sup.3+.
12. The cathode material according to claim 9, wherein an average
oxidation number of the nickel (2) is higher than 3.0 and is lower
than or equal to 3.5.
13. The cathode material according to claim 9, wherein the nickel
(2) contains Ni.sup.2+ and Ni.sup.3+.
14. The cathode material according to claim 9, wherein a ratio of
Ni.sup.3+ in the nickel (2) is 11 to 60%.
15. The cathode material according to claim 1, wherein the content
of Ni (Ni.sup.2+) sites in the total lithium site in the oxide
powder (b2) is lower than 5 mol %.
16. The cathode material according to claim 1, wherein the oxide
powder (a) is a monolithic particle and the oxide powder (b) is an
agglomerated particle composed of an agglomerate of micro
particles.
17. The cathode material according to claim 1, wherein the oxide
powder (a) has a D50 of 15 .mu.m or more and the oxide powder (b)
has a D50 of 8 .mu.m or less.
18. The cathode material according to claim 17, wherein the oxide
powder (a) has a D50 of 20 to 30 .mu.m and the oxide powder (b) has
a D50 of 4 to 7 .mu.m.
19. The cathode material according to claim 18, wherein 90% or more
of the oxide powder (b) is an agglomerate of micro particles having
a size of 1 to 4 .mu.m.
20. A lithium secondary battery comprising the cathode material
according to claim 1.
21. The lithium secondary battery according to claim 20, wherein
the lithium secondary battery is a pouch battery in which an
electrode assembly is sealed in a pouch-type case made of a
laminate sheet including a resin layer and a metal layer.
Description
TECHNICAL FIELD
[0001] The present invention relates to a cathode material with a
high energy density and a lithium secondary battery comprising the
same. More specifically, the present invention relates to a cathode
material comprising a mixture of an oxide powder (a) having a
specific composition and an oxide powder (b) having a specific
composition, wherein a mix ratio of the two oxide powders (oxide
powder (a): oxide powder (b)) is 50:50 to 90:10.
BACKGROUND ART
[0002] In recent years, chargeable and dischargeable secondary
batteries are widely used as energy sources of wireless mobile
equipment. Of these, lithium secondary batteries are generally used
due to advantages such as high energy density, discharge voltage
and power stability.
[0003] Lithium secondary batteries use metal oxide such as
LiCoO.sub.2 as a cathode material and carbon as an anode material
and are fabricated by inserting a polyolefin-based porous membrane
between an anode and a cathode and swelling a non-aqueous
electrolyte containing a lithium salt such as LiPF.sub.6.
LiCoO.sub.2 is commonly used as a cathode material for lithium
secondary batteries. LiCoO.sub.2 has several disadvantages of being
relatively expensive, having low charge/discharge capacity of about
150 mAh/g and unstable crystal structure at a voltage of 4.3 V or
higher and the risk of reacting with an electrolyte to cause
combustion. Furthermore, LiCoO.sub.2 is disadvantageous in that it
undergoes great variation in physical properties depending upon
variation in parameters in the preparation process thereof. In
particular, cycle properties and high-temperature storage
properties at high electric potential may be greatly varied
depending on partial variations of process parameters.
[0004] In this regard, methods to make batteries containing
LiCoO.sub.2 operate at high electric potential, such as coating the
outer surface of LiCoO.sub.2 with a metal (such as aluminum),
thermally treating LiCoO.sub.2, or mixing LiCoO.sub.2 with other
materials, have been suggested. Secondary batteries comprising such
a cathode material exhibit low stability at high electric potential
or have a limitation of application to mass-production.
[0005] In recent years, secondary batteries receive great attention
as power sources of electric vehicles (EVs), hydride electric
vehicles (HEV) or the like which are suggested as alternatives to
conventional gasoline vehicles, diesel vehicles or the like using
fossil fuels to solve air pollution caused thereby. Use of
secondary batteries is expected to further increase and the above
problems and problems associated with stability of batteries and
high-temperature storage properties at high electric potentials
arise.
[0006] In an attempt to solve the problems of LiCoO.sub.2, methods
using a mixture of two or more different lithium transition metal
oxides as a cathode material were suggested. These methods solve
the drawbacks of a cathode material in which the respective lithium
transition metal oxide is used singly.
[0007] However, conventional mixture-type cathode materials have a
limitation of the difficulty of obtaining superior synergetic
effects to the case of simple combination of two ingredients.
DISCLOSURE
Technical Problem
[0008] Therefore, the present invention has been made to solve the
above problems and other technical problems that have yet to be
resolved.
[0009] One object of the present invention is to provide a cathode
material exhibiting a high energy density and superior capacity
properties.
[0010] Another object of the present invention is to provide a
secondary battery which uses a cathode material exhibiting a high
energy density and thereby exert superior rate properties.
Technical Solution
[0011] In accordance with an aspect of the present invention, the
above and other objects can be accomplished by the provision of a
cathode material comprising a mixture of an oxide powder (a)
defined below and an oxide powder (b) selected from the group
consisting of an oxide powder (b1) defined below and an oxide
powder (b2) defined below and a combination thereof wherein a mix
ratio of the two oxide powders (oxide powder (a): oxide powder (b))
is 50:50 to 90:10.
[0012] [Oxide Powder (a)]
[0013] An oxide powder represented by Formula 1 below:
Li.sub.x(Co.sub.yA.sub.mD.sub.z)O.sub.t (1)
[0014] wherein 0.8.ltoreq.x.ltoreq.1.2, D.ltoreq.z.ltoreq.0.3,
1.8.ltoreq.t.ltoreq.4.2, (0.8-m-z).ltoreq.y.ltoreq.(2.2-m-z),
0.ltoreq.m.ltoreq.0.3, A is at least one selected from Mg and Ca,
and D is at least one selected from the group consisting of Ti, Zr,
Hf, V, Nb, Ta; Cr, Mo, W, Mn, Tc, Re, Fe, Ru, Os, Co, Rh, Ir, Ni,
Pd, Pt, Cu, Au, Ag, Zn, Cd, Hg, B, Al, Ga, In, TI, C, Si, Ge, Sn,
Pb, N, P, As, Sb and Bi;
[0015] [Oxide Powder (b1)]
[0016] An oxide powder represented by Formula 2a below:
Li.sub.x(Ni.sub.1-a-bMn.sub.aCo.sub.b).sub.yO.sub.2 (2a)
[0017] wherein 0.05.ltoreq.a.ltoreq.0.4, 0.1.ltoreq.b.ltoreq.0.4,
0.4.ltoreq.1-a-b.ltoreq.0.7, 0.95.ltoreq.x.ltoreq.1.05,
1.9.ltoreq.x+y.ltoreq.2.3;
[0018] [Oxide Powder (b2)]
[0019] An oxide powder which contains a transition metal mixture of
Ni, Mn and Co, has an average oxidation number of all transition
metals except for lithium, larger than +3 and satisfies Equations 3
and 4 below:
1.1<m(Ni)/m(Mn)<1.5 (3)
0.4<m(Ni.sup.2+)/m(Mn.sup.4+)<1 (4)
[0020] wherein m(Ni)/m(Mn) is a molar ratio of nickel to manganese
and (Ni.sup.2+)/m(Mn.sup.4+) is a molar ratio of Ni.sup.2+ to
Mn.sup.4+.
[0021] In the present invention, the cathode material is a mixture
of two types of lithium transition metal oxides, wherein the
mixture consists of the oxide powder (a) and the oxide powder (b)
at a ratio (weight ratio) of 50:50 to 90:10, thus exerting high
energy density and exhibiting superior capacity properties. A more
preferred mix ratio is 50:50 to 70:30.
[0022] As a result of intense and repeated research, the inventors
of the present invention discovered that, when the oxide powder (b)
is present in a mix ratio of 50% or less, lithium secondary
batteries can exert considerably superior discharge capacity
maintenance and desired rate properties at C-rate. Specifically,
when the cathode material according to the present invention is
used, lithium secondary batteries having a volumetric energy
density (VED) of 478 Wh/l or more and a gravimetric energy density
(GED) of 201 Wh/g can be fabricated.
[0023] On the other hand, when the oxide powder (b) is present in a
mix ratio exceeding 50%, discharge capacity considerably decreases,
and in particular, as C-rate increases, this decrease
disadvantageously becomes serious, and when the oxide powder (b) is
present in a mix ratio less than 10%, superior capacity properties
cannot be disadvantageously exerted.
[0024] The oxide powders may be surface-coated with a material such
as Al.sub.2O.sub.3 or mixed with Al.sub.2O.sub.3 to improve
properties thereof
[0025] The oxide powder (a) is for example preferably LiCoO.sub.2,
but the material is not limited thereto.
[0026] Of the oxide powder (b), the oxide powder (b1) satisfies a
specific composition defined by Formula (2a) (see FIG. 1), thus
exerting high capacity, superior cycle stability, superior storage
stability and high temperature stability. Hereinafter, the oxide
powder (b1) will be described in detail.
[0027] A total nickel molar ratio (1-a-b) is 0.4 to 0.7, an excess
amount, as compared to manganese and cobalt. When the content of
nickel is less than 0.4, high capacity cannot be expected, and when
the content exceeds 0.7, safety is disadvantageously greatly
deteriorated.
[0028] The content of the cobalt (b) is 0.1 to 0.4. When the
content of cobalt is excessively high (b>0.4), the overall cost
of raw materials increases and reversible capacity slightly
decreases due to the high content of cobalt. On the other hand,
when the content of cobalt is excessively low (b<0.1), both
sufficient rate properties and high powder density of batteries
cannot be accomplished.
[0029] When the content of lithium is excessively high (x>1.05),
in particular, safety may be disadvantageously deteriorated during
cycles at a high voltage (U=4.35 V) at T=60.degree. C. On the other
hand, when the content of lithium is excessively low (x<0.95),
rate properties are lowered and reversible capacity may
decrease.
[0030] In a preferred embodiment, in the oxide powder (b1), lithium
ions are intercalated and deintercalated between mixed transition
metal oxide layers ("MO layers"), some Ni ions derived from MO
layers are inserted into intercalation and deintercalation layers
of the lithium ions ("reversible lithium layer"), to bond the MO
layers.
[0031] Hereinafter, in this specification, Ni inserted into the
reversible lithium layer may also be referred to as an "inserted
Ni".
[0032] Specifically, there was a conventional concept in which, in
a case where some nickel moves downward from MO layers to the
reversible lithium layer and are fixed to the reversible lithium
layer, as shown in FIG. 2, the nickel will interfere with
intercalation and deintercalation of lithium. On the other hand,
the inventors of the present invention confirmed that, in this
case, it is possible to stabilize crystal structures and prevent a
problem in which the crystal structures are broken due to
intercalation and deintercalation of lithium.
[0033] Accordingly, it is possible to avoid additional structural
collapse by oxygen detachment, prevent further production of
Ni.sup.2+, improve both lifespan and safety, considerably improve
battery capacity and cycle properties, and afford desired rate
properties. The technical concept of the present invention is
considered to be a major advance that will completely usurp
conventional technology.
[0034] In the oxide powder (b1), Ni.sup.2+ and Ni.sup.3+ are
preferably present together in MO layers. Of these, some Ni.sup.2+
may be inserted into the reversible lithium layer. That is, the Ni
ions inserted into the reversible lithium layer are preferably
Ni.sup.2+.
[0035] This Ni.sup.2+ has a considerably similar size to lithium
ions (Li.sup.+) and is inserted into the reversible lithium layer,
thus blocking structural collapse caused by the repulsive force of
MO layers when lithium ions are intercalated during charging
without deforming crystal structures.
[0036] In addition, the Ni.sup.2+ is inserted between MO layers and
supports the same. The Ni.sup.2+ is contained in an amount capable
of stably supporting the space provided between MO layers and
thereby improving the desired charge stability and cycle stability.
In addition, Ni.sup.2+ is inserted in an amount which does not
interfere with intercalation and deintercalation of lithium ions in
the reversible lithium layer, to prevent deterioration in rate
properties. That is, when a molar fraction of Ni.sup.2+ inserted
into the reversible lithium layer is excessively high, the amount
of anions increases, and rock salt structures which have no
electrochemical reactivity are locally formed and interfere with
charging and discharging and thus cause a decrease in discharge
capacity.
[0037] Generally, taking into consideration the above points, a
molar fraction of Ni.sup.2+ inserted into the reversible lithium
layer is preferably 0.03 to 0.07, based on the total weight of
transition metals of the oxide powder (b1).
[0038] Meanwhile, as a ratio of Li to the transition metal (M)
(Li/M) decreases, the amount of Ni inserted into the MO layer
gradually increases. When an excessively great amount of Ni goes
downward to the reversible lithium layer, Ni interferes with
movement of Li+during charging and discharging, thus
disadvantageously decreasing reversible capacity or deteriorating
rate properties. On the other hand, when the ratio of Li/M is
excessively high, the amount of Ni inserted into the MO layer is
excessively small, thus disadvantageously causing structural
instability and deteriorating battery safety and lifespan.
Furthermore, in the case of excessively high Li/M value, the amount
of un-reacted Li.sub.2CO.sub.3 increases, that is, a great amount
of impurities is produced, thus causing deterioration of chemical
resistance and high-temperature stability. Accordingly, in a
preferred embodiment, a ratio of Li: M in LiNiMO.sub.2 may be
0.95:1 to 1.04:1.
[0039] In a preferred embodiment, the oxide powder (b1) does not
substantially contain a water-soluble base (in particular
Li.sub.2CO.sub.3) as an impurity.
[0040] Generally, nickel-based lithium-containing transition metal
oxide contains a great amount of water-soluble bases such as
lithium oxide, lithium sulfate, lithium carbonate and the like.
Such a water-soluble base may be firstly a base such as
Li.sub.2CO.sub.3 and LiOH present in LiNiMO.sub.2, and be secondly
a base produced by ion exchange (H.sup.+ (water) < - - -
>Li.sup.+ (surface, bulky outer surface)) on the surface of
LiNiMO.sub.2. The latter is commonly negligible.
[0041] The first water-soluble base is generally produced by
unreacted lithium material during sintering. The reason is that a
relatively great amount of lithium is added and sintered at a low
temperature to prevent collapse of layered crystal structures of
conventional nickel-based lithium-containing transition metal
oxides, and as a result, nickel-based lithium-containing transition
metal oxides have more grain boundaries, as compared to
cobalt-based oxides and lithium ions are not sufficiently reacted
during sintering.
[0042] On the other hand, as mentioned above, the oxide powder (b1)
stably maintains layered crystal structures, can be sintered at
relatively high temperatures under an air atmosphere and thus has
relatively few crystal grain boundaries. In addition, remaining of
unreacted lithium on particle surfaces is prevented and lithium
salts such as lithium carbonate and lithium sulfate are thus not
substantially present on the particle surfaces. In addition, in the
process of preparing the oxide powder (b1), addition of an excess
lithium source is unnecessary and a problem of formation of
impurities by the un-reacted lithium source left in a powder can be
fundamentally prevented.
[0043] As a result, many problems associated with presence of
water-soluble bases, in particular, problems in which decomposition
reaction of an electrolyte is accelerated at high temperatures to
produce a gas and thereby impair battery safety, can be solved.
Accordingly, lithium secondary batteries of the present invention
have advantages of superior storage stability, high temperature
stability and the potential for mass-production at a low cost.
[0044] Meanwhile, of the oxide powder (b), the oxide powder (b2)
provides considerably improved high rate charge and discharge
properties due to a superior layered crystal structure.
Hereinafter, the oxide powder (b2) will be described in detail.
[0045] As a preferred example, the layered crystal structures is an
.alpha.-NaFeO.sub.2 layered crystal structure.
[0046] It was known in the art that presence of Ni.sup.2+ and
Mn.sup.4+in equivalent amounts makes an average oxidation number of
transition metal ions +3 in order to obtain a desired layered
structure. However, since Ni.sup.2+ has a size substantially
similar to Li.sup.+, it moves to the lithium layer and readily
forms a sodium salt, thus disadvantageously causing deterioration
in electrochemical properties.
[0047] Accordingly, the inventors of the present invention
conducted a great deal of research to prepare a cathode active
material which has a stable layered crystal structure and exhibits
superior capacity and rate properties. As a result, the inventors
discovered that the stable layered crystal structure depends on the
size difference between the lithium ion and the transition metal
ion, rather than Ni.sup.2+ and Mn.sup.4+.
[0048] Specifically, the inventors confirmed that, in lithium
composite transition metal oxide having a layered crystal
structure, as size difference between the ions constituting the
reversible lithium layer and the MO layer (that is, lithium ions
and transition metal ions) increases, the two layers can be readily
separated and grown.
[0049] In this regard, use of metal elements having a small ion
size for the MO layer to increase size difference between ions may
be considered. However, this approach enables formation of the
desired layered structure, but has a limitation of relatively low
capacity due to decrease in the number of metal ions to transfer
electrons.
[0050] In this regard, the inventors of the present invention
attempted to accomplish the desired layered crystal structure
without causing deterioration in capacity. As a result, the
inventors confirmed that the size difference between the ions is
expressed by the bonding distance between each ion and the oxygen
ion or bonding force therebetween, and as a metal having cationic
characteristics has an increased oxidation number, it has a
decreased ionic radius. In addition, the inventors considered that
the difference between the MO layer and the lithium layer can be
increased by increasing the oxidation number of transition metals.
This expectation was confirmed through a great deal of
experimentation.
[0051] The idea that the layered crystal structure can be suitably
formed through increased size difference between the lithium ion
and the transition metal ion by increasing the average oxidation
number of the transition metal to a level higher than +3 is in
contrast to the conventional idea accepted in the art that the
average oxidation number of transition metals should be adjusted to
+3 to stabilize the layered crystal structure.
[0052] Meanwhile, in a case where the contents of Ni and Mn are
substantially equivalent in accordance with a conventional method,
Mn.sup.4+ ions induce formation of Ni.sup.2+ ions, and
disadvantageously, in a Mn-rich compound, a great amount of
Ni.sup.2+ is thus arranged in the lithium layer.
[0053] The inventors of the present invention predicted that the
best method to increase the oxidation number of transition metals
would be to adjust the total average oxidation number to +3 or
higher by decreasing the amount of Ni.sup.2+, which can be readily
permeated into the lithium layer. This prediction is based on the
idea that the amount of Ni.sup.3+ having a size smaller than
Ni.sup.2+ increases, thus causing an increase in size difference
between the ions.
[0054] Accordingly, the oxide powder (b2) contains nickel and
manganese wherein nickel is present in an amount higher than
manganese (See Equation (3)) and Ni.sup.2+ is present in an amount
smaller than Mn.sup.4+ (See Equation (4)). Specifically, the oxide
powder (b2) is a lithium nickel-manganese-cobalt oxide wherein (i)
an average oxidation number of nickel-manganese-cobalt, all
transition metals except for lithium is larger than +3, (ii) nickel
is present in an amount larger than manganese and (iii) Ni.sup.2+
is present in an amount smaller than Mn.sup.4+.
[0055] Such a lithium nickel-manganese-cobalt oxide maintains an
average oxidation number of transition metals at a level larger
than +3, thus considerably decreasing the amount of transition
metals present in the reversible lithium layer based on the stable
crystal structure of the cathode material and improving mobility of
lithium ions and rate properties as well as capacity.
[0056] Regarding the aspect (i), the oxide powder (b2) has an
average oxidation number of transition metals except lithium,
higher than +3, thus decreasing an average size of transition metal
ions, increasing the size difference between lithium ions, and
promoting separation between layers, thereby forming a stable
layered crystal structure.
[0057] When the average oxidation number of transition metals is
excessively increased, electric charges capable of transferring
lithium ions are decreased, thus disadvantageously decreasing
capacity. Preferably, the average oxidation number of transition
metals is higher than 3.0 and not higher than 3.5, more preferably,
3.01 to 3.3, more particularly preferably, 3.1 to 3.3.
[0058] In this case, the total average oxidation number of Mn and
Ni corresponding thereto is 3.0 to 3.5, preferably, 3.1 to 3.3.
[0059] As herein used, the expression "average oxidation number of
transition metals except for lithium" means, that, for example, an
average oxidation number of lithium ions is not considered,
although some lithium ions are contained in the site of transition
metals.
[0060] Control of average oxidation number of transition metals is
for example carried out by controlling a ratio of a transition
metal of a transition metal precursor and the amount of a lithium
precursor reacted in the process of preparing lithium transition
metal oxide.
[0061] Regarding the aspect (ii), the oxide powder (b2) is composed
of a material in which a molar ratio of nickel to manganese is
higher than 1.1 and lower than 1.5, as represented by Equation (3)
below.
1.1<m(Ni)/m(Mn)<1.5 (3)
[0062] As such, when nickel is present in an larger amount than
manganese, nickel in an amount corresponding to the difference
between the nickel content and the manganese content, is changed to
Ni.sup.3+, thus decreasing ion size. Accordingly, the average size
difference between the lithium ion and the transition metal ion
increases, thus minimizing permeation of Ni.sup.2+ into the lithium
layer and improving stability of the layered crystal structure.
[0063] When m(Ni)/m(Mn) is larger than 1.5, disadvantageously,
safety is deteriorated and preparation cost of active materials
increases due to decreased Mn content. In a more preferred
embodiment, the ratio of m(Ni)/m(Mn) may be 1.2 to 1.4.
[0064] On the other hand, although the content of manganese is
larger than that of nickel, in a case where an average oxidation
number of transition metals is +3 or higher, the layered crystal
structures are formed, but +4 ions which do not contribute to
charging/discharging are increased and capacity is thus
decreased.
[0065] As mentioned above, in the case where the oxide powder (b2)
according to the present invention contains excess nickel, as
compared to manganese, the nickel is composed of nickel (1) present
in an excessive amount, as compared to the manganese content and
nickel (2) present in an amount corresponding to the manganese
content.
[0066] The nickel has an average oxidation number higher than
2+.
[0067] Preferably, the nickel (1) is Ni.sup.3+, and the nickel (2)
includes both Ni.sup.2+ and Ni.sup.3+.
[0068] Of the nickel (2) including Ni.sup.2+ and Ni.sup.3+, a ratio
of Ni.sup.3+ is preferably 11 to 60%. That is, when the ratio is
lower than 11%, desired electrochemical properties cannot be
obtained, and when the ratio is higher than 60%, variation in
oxidation number is excessively small, thus disadvantageously
increasing a capacity decrease and a dispersed lithium amount. In
this case, the average oxidation number of manganese and nickel is
about 3.05 to about 3.35.
[0069] Regarding the aspect (iii), the oxide powder (b2) is
composed of a material in which a molar ratio of Ni.sup.2+ to
Mn.sup.4+ is higher than 0.4 and lower than 1, as represented by
Equation (4). That is, Ni.sup.2+ and Mn.sup.4+ are not present in
equivalent amounts, but Ni.sup.2+is present in a smaller amount
than Mn.sup.4+.
0.4<m(Ni.sup.2+)/m(Mn.sup.4+)<1 (4)
[0070] When the ratio of m(Ni.sup.2+)/m(Mn.sup.4+) is 1 or higher,
the average oxidation number of transition metals does not increase
and ion size difference thus cannot be induced, and when the ratio
is 0.4 or lower, the oxidation number of transition metals is
excessively high, capacity is deteriorated due to decrease in
amount of movable electric charges. When the ratio of
m(Ni.sup.2+)/m(Mn.sup.4+) is higher than 0.4 and is equivalent to
or lower than 0.9, considerably superior electrochemical properties
can be obtained.
[0071] In the oxide powder (b2), the content of cobalt (Co) in
transition metals may be lower than 10 mol % of the total
transition metal content. An increase in cobalt content causes
disadvantages of cost increase and unstable Co.sup.4+ and low
stability during charging.
[0072] As mentioned above, in the oxide powder (b2), since nickel
is present in an amount higher than manganese and an average
oxidation number of transition metals is higher than +3, the size
difference between the lithium ions and the transition metal ions
increases, layer separation is accelerated, and insertion of
Ni.sup.2+ into the lithium layer can be minimized. In the cathode
material, the content of nickel inserted into lithium sites is
lower than 5 mol %, as a ratio of Ni(Ni.sup.2+) sites with respect
to the total Li sites.
[0073] Nickel, manganese and cobalt, as transition metals present
in the oxide powder (b2) may be partially substituted by other
metal element (s), and preferably by other metal (s), anionic
element (s) or the like in a small amount of 5% or less so long as
they maintain layered crystal structures. Obviously, this case is
within the scope of the present invention so long as the features
of the present invention are satisfied.
[0074] Meanwhile, the oxide powder (a) preferably may have a
monolithic structure. Accordingly, the oxide powder (a) has no or
little inner porosity and exhibits improved stability of crystal
particles, as the size of particles increases, thus enabling easy
manufacture of batteries comprising the same and improving
manufacturing process efficiency.
[0075] For example, the oxide powder (a) is a potato shaped
monolithic particle and may have D50 of 10 .mu.m or more,
preferably 15 .mu.m or more.
[0076] Meanwhile, the oxide powder (b) preferably has an
agglomerated structure, that is, a form of an agglomerate of micro
powders and may have an inner porosity. Such an agglomerated
particle structure maximizes a surface area which reacts with an
electrolyte, thus exerting high rate properties and increasing
reversible capacity of the cathode.
[0077] For example, the agglomerated oxide powder (b) is in the
form of an agglomerate of microparticles of 1 .mu.m to 5 .mu.m and
has D50 of 10 .mu.m or less, preferably 8 .mu.m to or less, more
preferably 4 to 7 .mu.m. Particularly preferably, an agglomerate of
90% or more of microparticles having a size of 1 to 4 .mu.m (D50)
may constitute an oxide powder (b).
[0078] The present invention also provides a lithium secondary
battery comprising the cathode material. Generally, the lithium
secondary battery comprises a cathode, an anode, a separator
interposed between the electrodes and a lithium-containing
non-aqueous electrolyte.
[0079] For example, the cathode is prepared by applying a cathode
mix comprising a cathode active material, a conductive material and
a binder to a cathode current collector, followed by drying and
pressing. The cathode mix may further comprise a filler, if
necessary.
[0080] The cathode current collector is generally produced to have
a thickness of 3 to 500 .mu.m. There is no particular limit to the
cathode current collector, so long as it has suitable conductivity
without causing adverse chemical changes in the produced battery.
Examples of the cathode current collector include stainless steel,
aluminum, nickel, titanium, sintered carbon, and aluminum or
stainless steel surface-treated with carbon, nickel, titanium,
silver or the like. If necessary, these current collectors may also
be processed to form fine irregularities on the surface thereof so
as to enhance adhesion to the cathode active materials. In
addition, the current collectors may be used in various forms
including films, sheets, foils, nets, porous structures, foams and
non-woven fabrics.
[0081] The conductive material is commonly added in an amount of 1
to 30% by weight, based on the total weight of the mixture
including the cathode active material. Any conductive material may
be used without particular limitation so long as it has suitable
conductivity without causing adverse chemical changes in the
produced secondary battery. Examples of conductive materials that
can be used in the present invention include graphite such as
natural or artificial graphite; carbon black such as carbon black,
acetylene black, Ketjen black, channel black, furnace black, lamp
black and thermal black; conductive fibers such as carbon fibers
and metallic fibers; metallic powders such as carbon fluoride
powder, aluminum powder and nickel powder; conductive whiskers such
as zinc oxide and potassium titanate; conductive metal oxides such
as titanium oxide; and polyphenylene derivatives.
[0082] The binder is a component which enhances binding of an
electrode active material to a conductive material and current
collector. The binder is commonly added in an amount of 1 to 30% by
weight, based on the total weight of the compound including the
anode active material. Examples of the binder include
polyvinylidene, polyvinyl alcohol, carboxymethylcellulose (CMC),
starch, hydroxypropylcellulose, regenerated cellulose, polyvinyl
pyrollidone, tetrafluoroethylene, polyethylene, polypropylene,
ethylene propylene diene terpolymer (EPDM), sulfonated EPDM,
styrene butadiene rubber, fluororubbers and various copolymers.
[0083] The filler is a component used to inhibit expansion of the
cathode. There is no particular limit to the filler, so long as it
does not cause adverse chemical changes in the produced battery and
is a fibrous material. As examples of the filler, there may be used
olefin polymers such as polyethylene and polypropylene; and fibrous
materials such as glass fibers and carbon fibers.
[0084] For example, the anode is prepared by applying an anode
active material to an anode current collector, followed by drying.
The anode active material may further comprise the afore-mentioned
ingredients, i.e., the conductive material, the binder and the
filler.
[0085] The anode current collector is generally produced to have a
thickness of 3 to 500 .mu.m. There is no particular limit to the
anode current collector, so long as it has suitable conductivity
without causing adverse chemical changes in the produced battery.
Examples of the anode current collector include copper, stainless
steel, aluminum, nickel, titanium, sintered carbon, and copper or
stainless steel surface-treated with carbon, nickel, titanium or
silver, and aluminum-cadmium alloys. Similar to the cathode current
collector, if necessary, these current collectors may also be
processed to form fine irregularities on the surface thereof so as
to enhance adhesion to the anode active materials. In addition, the
current collectors may be used in various forms including films,
sheets, foils, nets, porous structures, foams and non-woven
fabrics.
[0086] In addition, examples of anode active materials that can be
used in the present invention include carbons such as hard carbons
and graphite carbons; metal composite oxides such as
Li.sub.yFe.sub.2O.sub.3(0.ltoreq.y.ltoreq.1),
Li.sub.yWO.sub.2(0.ltoreq.y.ltoreq.1),
Sn.sub.xMe.sub.1-xMe'.sub.yO.sub.z (Me: Mn, Fe, Pb, Ge; Me': Al, B,
P, Si, Group I, II and III elements of the Periodic Table,
halogens; 0<x.ltoreq.1; 1.ltoreq.y.ltoreq.3;
1.ltoreq.z.ltoreq.8); lithium metals; lithium alloys; silicon-based
alloys; tin-based alloys; metal oxides such as SnO, SnO.sub.2, PbO,
PbO.sub.2, Pb.sub.2O.sub.3, Pb.sub.3O.sub.4, Sb.sub.2O.sub.3,
Sb.sub.2O.sub.4, Sb.sub.2O.sub.5, GeO, GeO.sub.2, Bi.sub.2O.sub.3,
Bi.sub.2O.sub.4, Bi.sub.2O.sub.5 and the like; conductive polymers
such as polyacetylene, Li--Co--Ni materials and combinations
thereof.
[0087] The separator is an insulating thin film having high ion
permeability and mechanical strength and typically has a pore
diameter of 0.01 to 10 .mu.m and a thickness of 5 to 300 .mu.m.
Examples of useful separators include, but are not limited to
polymers having a microporous structure such as polyethylene,
polypropylene, polytetrafluoroethylene, polyethylene terephthalate,
polybutylene terephthalate, polyethylene naphthalate and
combinations thereof.
[0088] In addition, one side or two sides of the separator may be
coated with an inorganic material.
[0089] The lithium-containing non-aqueous electrolyte is composed
of a non-aqueous electrolyte and a lithium salt.
[0090] Examples of the non-aqueous electrolytic solution include
non-protic organic solvents such as N-methyl-2-pyrollidinone,
propylene carbonate, ethylene carbonate, butylene carbonate,
dimethyl carbonate, diethyl carbonate, gamma-butyrolactone,
1,2-dimethoxy ethane, tetrahydroxy franc, 2-methyl tetrahydrofuran,
dimethylsulfoxide, 1,3-dioxolane, formamide, dimethylformamide,
dioxolane, acetonitrile, nitromethane, methyl formate, methyl
acetate, phosphoric acid triester, trimethoxy methane, dioxolane
derivatives, sulfolane, methyl sulfolane,
1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives,
tetrahydrofuran derivatives, ether, methyl propionate, ethyl
propionate and combinations thereof.
[0091] Preferably, a combination of one or more of a cyclic
carbonate solvent and a linear carbonate solvent is used.
[0092] In addition, preferably, ethylene carbonate or a combination
of ethylene carbonate and ethylmethylcarbonate is used.
[0093] The lithium salt is a material that is readily soluble in
the above-mentioned non-aqueous electrolyte and may include, for
example, LiCl, LiBr, LiI, LiClO.sub.4, LiBF.sub.4,
LiB.sub.10Cl.sub.10, LiPF.sub.6, LiCF.sub.3SO.sub.3,
LiCF.sub.3CO.sub.2, LiAsF.sub.6, LiSbF.sub.6, LiAlCl.sub.4,
CH.sub.3SO.sub.3Li, CF.sub.3SO.sub.3Li,
(CF.sub.3SO.sub.2).sub.2NLi, chloroborane lithium, lower aliphatic
carboxylic acid lithium, lithium tetraphenyl borate, imide and a
combination thereof
[0094] As the non-aqueous electrolyte, an organic or inorganic
solid electrolyte may be used.
[0095] Examples of the organic solid electrolyte include
polyethylene derivatives, polyethylene oxide derivatives,
polypropylene oxide derivatives, phosphoric acid ester polymers,
polyagitation lysines, polyester sulfide, polyvinyl alcohol, poly
(vinylidene fluoride) and polymers containing ionic dissociations
groups.
[0096] Examples of the inorganic solid electrolyte include lithium
nitrides, lithium halogenides and lithium sulfates such as
Li.sub.3N, LiI, Li.sub.5NI.sub.2, Li.sub.3N--LiI--LiOH,
LiSiO.sub.4, LiSiO.sub.4--LiI--LiOH, Li.sub.2SiS.sub.3,
Li.sub.4SiO.sub.4, Li.sub.4SiO.sub.4--LiI--LiOH and
Li.sub.3PO.sub.4--Li.sub.2S--SiS.sub.2.
[0097] Additionally, in order to improve charge/discharge
characteristics and flame retardancy, for example, pyridine,
triethylphosphite, triethanolamine, cyclic ether, ethylenediamine,
n-glyme, hexaphosphoric triamide, nitrobenzene derivatives, sulfur,
quinone imine dyes, N-substituted oxazolidinone, N,N-substituted
imidazolidine, ethylene glycol dialkyl ether, ammonium salts,
pyrrole, 2-methoxy ethanol, aluminum trichloride or the like may be
added to the non-aqueous electrolyte. If necessary, in order to
impart incombustibility, the non-aqueous electrolyte may further
include halogen-containing solvents such as carbon tetrachloride
and ethylene trifluoride. Further, in order to improve
high-temperature storage characteristics, the non-aqueous
electrolyte may additionally contain carbon dioxide gas.
[0098] In a preferred embodiment, the secondary battery may be a
pouch battery in which an electrode assembly is sealed in a
pouch-type case made of a laminate sheet including a resin layer
and a metal layer.
[0099] For example, the laminate sheet may have a structure
including an inner resin layer, a blocking metal layer and an outer
resin layer. The outer resin layer should have tensile strength and
weatherability equal to or higher than a predetermined level in
order to secure superior resistance to external environments. In
this regard, the polymer resin for the outer resin layer is
preferably a polyethylene terephthalate (PET) and drawn nylon film.
The blocking metal layer is preferably aluminum to prevent
incorporation and leakage of foreign materials such as gas and
humidity and improve strength of the battery case. The polymer
resin for the inner resin layer is preferably a polyolefin resin
which has thermal fusion (thermal adhesion) and low absorbance in
order to inhibit invasion of the electrolyte and is not swollen or
precipitated by the electrolyte, more preferably undrawn
polypropylene (CPP).
BRIEF DESCRIPTION OF THE DRAWINGS
[0100] The above and other objects, features and other advantages
of the present invention will be more clearly understood from the
following detailed description taken in conjunction with the
accompanying drawings, in which:
[0101] FIG. 1 is a schematic view illustrating the crystal
structure of an oxide powder (b) according to one embodiment of the
present invention;
[0102] FIG. 2 is a graph showing a preferred composition range of
an oxide powder (b) according to one embodiment of the present
invention; and
[0103] FIG. 3 is a graph showing a discharge capacity ratio in
Experimental Example 1.
BEST MODE
[0104] Now, the present invention will be described in more detail
with reference to the following examples. These examples are
provided only for illustrating the present invention and should not
be construed as limiting the scope and spirit of the present
invention.
PREPARATION EXAMPLE 1
[0105] Preparation of Oxide Powder (b)
[0106] Mixed hydroxide MOOH
(M=Ni.sub.4/15(Mn.sub.1/2Ni.sub.1/2).sub.8/15Co.sub.0.2) was used
as a mixed transition metal precursor, the mixed hydroxide was
mixed with Li.sub.2CO.sub.3 at a stoichiometric ratio
(Li:M=1.02:1), and the mixture was sintered in air at 920.degree.
C. for 10 hours to prepare
LiNi.sub.0.53Co.sub.0.2Mn.sub.0.27O.sub.2. At this time, secondary
particles did not collapse and were still maintained.
[0107] It could be confirmed by X-ray analysis that all samples had
well-grown layer crystal structures. In addition, unit cell volume
did not considerably vary as sintering temperature increased. This
means that considerable oxygen deficiency and considerable increase
in anion mixing did not occur and evaporation of lithium
substantially did not occur.
[0108] It was confirmed that
LiNi.sub.0.53Co.sub.0.2Mn.sub.0.27O.sub.2 has a structure in which
nickel is incorporated in about 3.9 to about 4.5% in a reversible
lithium layer and a suitable amount of Ni.sup.2+ is incorporated in
the reversible lithium layer, thus exhibiting structural
stability.
EXAMPLE 1
[0109] LiCoO.sub.2 having a monolithic structure and D50 of about
15 to about 20 .mu.m and LiNi.sub.0.53Co.sub.0.2Mn.sub.0.27O.sub.2
having D50 of about 5 to 8 .mu.m, as an agglomerate of micro
particles a size of about 1 to about 2 .mu.m obtained in
Preparation Example 1-1 were mixed at a ratio of 50:50 to prepare a
cathode material mix.
[0110] The cathode material mix, Super P as a conductive material
and polyvinylidene fluoride as a binder were mixed at a weight
ratio 92:4:4, and N-methyl pyrrolidone (NMP) was added thereto to
prepare a slurry. The cathode slurry was applied to an aluminum
collector, followed by drying in a vacuum oven at 120.degree. C. to
produce a cathode.
[0111] In addition, mesocarbon microbead (MCMB) as an anode active
material, super P as a conductive material and PVdF as a binder
were mixed at a weight ratio of 92:2:6, followed by dispersion in
NMP and coating on a copper foil, to produce an anode.
[0112] A porous membrane made of polypropylene was inserted between
the anode and cathode thus obtained to manufacture an electrode
assembly. The electrode assembly was added to a pouch-type case, an
electrode lead was connected, and a solution consisting of ethylene
carbonate (EC) and dimethyl carbonate (DMC) (1:1, volume ratio)
containing 1M LiPF.sub.6 was inserted as an electrolyte, followed
by sealing to assemble a lithium secondary battery.
EXAMPLE 2
[0113] A cathode material mix was prepared and a lithium secondary
battery was produced in the same manner as in Example 1 except that
a weight ratio of LiCoO.sub.2 and
LiNi.sub.0.53Co.sub.0.2Mn.sub.0.27O.sub.2 in the cathode material
mix was 70:30.
COMPARATIVE EXAMPLE 1
[0114] A cathode material mix was prepared and a lithium secondary
battery was produced in the same manner as in Example 1 except that
a weight ratio of LiCoO.sub.2 and
LiNi.sub.0.53Co.sub.0.2Mn.sub.0.27O.sub.2 in the cathode material
mix was 40:60.
COMPARATIVE EXAMPLE 2
[0115] A cathode material mix was prepared and a lithium secondary
battery was produced in the same manner as in Example 1 except that
LiNi.sub.1/3Co.sub.1/3Mn.sub.1/3O.sub.2 was used instead of
LiNi.sub.0.53Co.sub.0.2Mn.sub.0.27O.sub.2.
EXPERIMENTAL EXAMPLE 1
[0116] Discharge capacity (1C rate charge) of batteries produced in
Examples 1 and 2 and the battery produced in Comparative Example 1
were measured at 0.2C, 0.5 C, 1C, 1.5C and 2C rate and a ratio of
discharge capacity at each C-rate with respect to 5C rate capacity
was calculated. The results thus obtained are shown in FIG. 3.
[0117] As can be seen from FIG. 3, discharge capacity of the
battery of Comparative Example 2 rapidly decreases, as C-rate
increases, and on the other hand, batteries of Examples 1 and 2 of
the present invention exhibited considerably superior C-rate
properties, and in particular, the battery of Example 2 containing
30% of oxide (b) exhibited superior C-rate properties in which
discharge capacity is as high as 90% or more at a 2C rate. In
addition, it can be seen that this improvement in C-rate properties
was exhibited even at a low C-rate of 1C, and batteries of Examples
1 and 2 exhibited more considerable improvement in discharge
properties, as C-rate thereof increases.
[0118] As apparent from the fore-going, use of combination of
lithium cobalt oxide and lithium nickel manganese cobalt oxide
alone cannot exhibit desired rate properties, and when a material
having a predetermined composition is mixed with these substances
at a specific mix ratio, synergetic effects can be obtained.
INDUSTRIAL APPLICABILITY
[0119] As apparent from the above description, the secondary
battery of the present invention can exert high energy density as
well as high capacity by using a combination of an oxide powder (a)
having a specific composition and an oxide powder (b) having a
specific composition, as a cathode material and by controlling the
mix ratio of these oxide powders to a predetermined range. In
particular, the oxide powder (b) has a stable layered structure and
thus improves stability of crystal structures during charge and
discharge. Accordingly, the battery containing this cathode
material has advantages of high capacity, superior cycle stability
and improved overall battery performance.
[0120] Although the preferred embodiments of the present invention
have been disclosed for illustrative purposes, those skilled in the
art will appreciate that various modifications, additions and
substitutions are possible, without departing from the scope and
spirit of the invention as disclosed in the accompanying
claims.
* * * * *